As used herein and in the claims, the term "composite arch structure" is intended to include arch structures having any one of a number of cross sectional configurations, well known in the art, such as circular, pipe arch, vertical elipse, horizontal elipse, underpass, arch, low profile arch, high profile arch and inverted pear.
In recent years, conventional rigid arch designs have been superseded by relatively flexible designs utilizing flexible retaining wall structures similar to those described in U.S. Pat. No. 3,282,056. The strength of these structures is derived primarily from the backfill material located thereabout. The structure, made up of curved, corrugated sheets, must have sufficient strength to be capable of self support during installation. The strength of the metallic structure, on the other hand, is not sufficient to support the superimposed load after installation. While its strength must be adequate to carry its share of the superimposed load after installation, the backfill material is intended to be the principle load bearing and transmitting element of the finished structure.
The design features of structures of this sort, utilizing the composite arch principle, are dependent upon the shear and compression values of the backfill material, the proper related curvatures of the flexible lining and the type of backfill material enveloping the underground structure when finished. So long as the dimensions of the composite arch structure remain relatively small, no difficulty is encountered in the backfilling procedure.
More recently, prior art workers have turned their attention to the construction of so called "long-span" composite arch structures, generally defined as having a span greater than from about 15 feet to about 25 feet (from about 4.6 m to about 7.6 m) and a minimum radius of curvature of from about 8 feet to about 12 feet (from about 2.4 m to about 3.7 m). Examples of such long-span composite arch structures are taught, for example, in U.S. Pat. Nos. 3,508,406 and 3,735,595.
Long-span structures are characterized by certain difficulties generally not encountered with the smaller structures. For one thing, they have less initial stability until supported by the backfill material and the backfilling procedure is far more critical. For example, as the backfill progresses upwardly along the flexible retaining wall portions of the composite arch structure, the top arch portion tends to shift upwardly at its center or "peak." To overcome this problem, the center part of the top arch portion may be loaded or held in place and in shape internally by frame members, cables or the like. Further difficulties are encountered when backfilling and compacting the soil along or around the junction lines between the flexible retaining walls (which have a relatively sharp curvature and are situated substantially vertically), and the flexible arch which extends therebetween. As the compaction proceeds, the horizontal component of the load becomes greater than the vertical component, thus causing distortion of the structure which can only be avoided by extremely careful backfilling from both sides.
Prior art workers have developed a number of expedients to overcome these difficulties. For example, the composite arch structure may be provided with open-top bins located along the upper surface of the liner. Backfill material is compacted in layers in the bins and around the liner, the bins serving to confine, reinforce and strengthen the compacted backfill, as well as acting as stiffeners for the top arch portion of the liner to reduce initial peaking and subsequent flattening. Such a structure is taught in the above mentioned U.S. Pat. No. 3,735,595.
Another expedient is to provide circumferential rib stiffeners about the liner. These rib stiffeners provide increased stiffness to reduce peaking during backfilling. They further reduce local buckling and excessive flattening during the remainder of the backfilling procedure.
Yet another expedient is set forth in the above mentioned U.S. Pat. No. 3,508,406 wherein longitudinally extending load spreading buttress means are provided on the composite arch structure, located to either side of the vertical axis thereof at positions where the radial force acting on the structure forms an angle of about 45.degree. or more to the horizontal. These buttress means anchor the base of the top arch portion of the structure and provide lengths of consolidated material at the locations where compaction and backfilling equipment cannot effectively work, enabling the backfilling procedure to continue without distortion of the structure. For very wide arches, one or more stiffening members extending between the buttress means and over the top arch portion of the structure may be provided, the top arch portion of the structure being affixed to the stiffening members.
The American Association of State Highway and Transportation Officials (AASHTO) has devised a series of standard specifications for highway bridges, including long-span composite arch structures of the type to which the present invention is directed. To date, such structures have been built with spans up to 51 feet (16 m). It is presently generally accepted that the top arch portion of such structures is limited to from about a 60.degree. to about an 80.degree. central angle.
AASHTO Standards also set forth the minimum amount of cover or backfill to be located over the structure in order for the structure to perform properly. Where less than minimum overhead cover is applied, loads are not properly distributed through the soil and the soil or backfill does not sustain its preponderant share of the load. For example, under live load such as that imposed by a vehicle, failure can occur because this load is localized and applied to the area of the arch immediately below the point of load application. In situations where only minimum or less than minimum backfill can be applied to the top arch portion of the liner structure, prior art workers have provided an elongated slab of reinforced concrete located above the liner structure and near or immediately below the surface of the road extending across the shallow backfill cover. The elongated slab extends substantially the length of the liner structure and serves as a load spreading device.
The present invention is based upon the discovery that if, in a long span composite arch structure, a longitudinally extending stiffening element is structurally connected to the center of the top arch structure, extending substantially, the length of the structure, a number of advantages are obtained. First of all, the stiffening element, being structurally connected to the center of the top arch portion of the liner structure, serves as an arch "interrupter." In other words, that portion of the arch to which the stiffening element is connected is, itself, stiffened. The remainder of the arch structure remains flexible, capable of yielding to develop adequate soil arching. Nevertheless, the central angle of the structure has been subdivided into two lesser angles, as has the cord of the top arch portion. As a result, the top arch portion has been additionally rigidified. The top arch portion rigidity is approximately an inverse function of the square of the cord length or the angles subtended by the top arch portion and the segments into which it is divided. As a result of this, through the practice of the present invention the central angle of long span structures can safely be increased up to 90.degree. or more and the span width may be increases up to about 60 feet.
Furthermore, the stiffening element can serve as top weighting for the structure, minimizing or preventing peaking during the backfilling operation. The stiffening element will serve as a live, thermal and dead load distributor, providing a sound structure even in circumstances where less than minimum recommended backfill cover must be used.